I have an rb pro sd so I can't say what's involved to put the 6x8 back on the prior rbs, but consider this: the pro sd bodies are rather inexpensive, and the backs are in plentiful supply... or at least they were last time I checked. But the caveat is that you can't see the whole recorded image on the ground glass. And the final image you get is a few mm short of 6x8. So you don't gain all that much relative to 6x7. Nevertheless I do find the 6x8 back very useful, I like the ratio.
You can also get a (rare) 545i back for the rb, and shoot quickloads with that. It wastes a fair amount of film but you get big, luscious squares and the film is very flat and can be developed one shot at a time, which is sometimes very helpful.
You can also use the mamiya double cut film/plate holder "type J" on the rb; I shoot 3.25x2.25" sheet film on the rb that way. I have some other backs called "type A" which work in the same way. Yes, you can shoot plates on the rb, there is a good back for that!
Gotta love this rig eh.
I also have an rz pro2 and like it, but have been using a (depinned) rz t&s adapter on the rb, just because of the 6x8 back and all the other goodies I have for the rb.
